8 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



The structure of the polypeptides is known from their synthesis, 

 but a study of their properties has made it appear probable that other 

 structures can occur. Their nomenclature in the case of optically 

 active compounds is simple, e.g., 1-leucyl-d-alanyl-l-tyrosine, but where 

 racemic forms are concerned it is more difficult ; before their configura- 

 tion was established Fischer had simply labelled them as the A and 

 B forms, the A form denoting the more insoluble. 



In their properties the polypeptides closely resemble ; the proteins : 

 the biuret reaction is positive as soon as the compounds contain three 

 units and the other colour reactions are positive if the compound con- 

 tains the particular unit : the higher polypeptides are precipitated by 

 tannic acid, phosphotungstic acid and other alkaloidal reagents : some 

 can also be salted out by saturation with ammonium sulphate. The 

 octadecapeptide has the highest molecular weight of any compound as 

 yet obtained by synthesis, and had it been discovered in nature it 

 would undoubtedly have been regarded as a protein. 



Not only do the polypeptides closely resemble the proteins in 

 properties but also in their behaviour to enzymes. The majority of 

 the polypeptides are readily hydrolysed by the proteoclastic enzymes, 

 with the exception of pepsin. Several interesting and remarkable 

 factors have been observed in this connection ; only those polypeptides 

 containing the natural isomer of the amino acid are hydrolysed, and in 

 the case of racemic compounds the hydrolysis is effected asymmetri- 

 cally ; the hydrolysis also depends upon the order of combination of 

 the units, especially in the case of the enzyme trypsin. 



The Structure of Proteins. 



The close resemblance of the synthetical polypeptides to the 

 natural proteins shows most conclusively that the protein molecule is 

 built up of amino acids combined together in the form of acid amides, 

 that they have in fact the following structure : 



NH, . CHR . CO (NH . CHR . CO)x NH . CHR . COOH. 



The arrangement of the amino acids in the synthetical polypeptides 

 is known by their method of preparation. Isomers occur amongst the 

 dipeptides, e.g., glycyl-alanine and alanyl-glycine. The total number 

 of possible dipeptides is 272, and this number increases as we proceed 

 to consider tri-, tetra-, penta-peptides, etc. On account of this large 

 number of isomers amongst even simple polypeptides it is not con- 

 ceivable how a natural protein can be synthesised, except by chance, 

 until it has been ascertained in what order the various units are com- 

 bined with one another. The action of trypsin upon* the dipeptides 



